server: support multiple generations from one prompt (OAI "n" option) (#17775)

* backend support

* server: support multiple generations from one prompt (OAI "n" option)

* fix invalid batch

* format oai

* clean up

* disable ctx shift

* add test

* update comments

* fix style

* add n_cmpl to docs [no ci]

* allowing using both n_cmpl and n

tools/server/server-common.h

@@ -215,6 +215,8 @@ public:
                 llama_pos pos,
                 int32_t seq_id,
                 size_t & n_tokens_out) const;
+
+    server_tokens clone() const;
 };